1. Field of the Invention
The present invention relates to an image forming apparatus such as a color printer, and more particularly, to an image forming apparatus which can form an image by superposing a plurality of images on one transfer medium.
2. Description of the Related Art
A practical example of the image forming apparatus of this type is disclosed in Published Unexamined Japanese Patent Application No. 64-28665.
In this apparatus, a charging means, an exposure means, a developing means for developing agents (toners) of four colors, yellow, magenta, cyan, and black, a transfer drum, and a cleaning means are arranged so that each faces a one photoreceptor. As the photoreceptor makes one revolution, an image of one color is formed on the photoreceptor and transferred to a transfer medium, which is wound on the transfer drum. As the photoreceptor and the transfer drum make four revolutions, images of four colors are superposed on one and the same transfer medium.
Thus, in the conventional multi-color image forming apparatus, the images of four colors cannot be superposed on the same transfer medium unless the medium wound on the transfer drum is caused to make four revolutions, so that high-speed processing cannot be effected.
Thereupon, apparatuses capable of high-speed multi-color image forming have been proposed, such as disclosed in Published Unexamined Japanese Patent Applications Nos. 64-40847, 64-44457, and 64-49062.
In these apparatuses, a plurality of image forming means for forming images of different colors on an image carrier are successively arranged facing a transportation path for a transfer medium. A multi-color image can be formed as the transfer medium is transported past the individual image forming means.
According to these prior art apparatuses, however, a plurality of colors are simply superposed in succession by transporting the transfer medium past the individual image forming means. Therefore, the developing agent transferred to the transfer medium by means of a preceding image forming means is electrically charged and changes its polarity, due to the influence of corona discharge in a transfer process using a subsequent image forming means. In the transfer process using the subsequent image forming means, the developing agent transferred to the transfer medium by means of the preceding image forming means is separated from the medium and scatters, so that satisfactory images cannot be formed. This constitutes a hindrance to the practical use of the proposed apparatuses.
Thus, there have conventionally been high-speed image forming apparatuses in which a plurality of image forming means for forming images of different colors on an image carrier are successively arranged facing a transportation path for a transfer medium. A multi-color image can be formed as the transfer medium is transported past the individual image forming means. In a transfer process using a subsequent image forming means, however, a developing agent transferred to the transfer medium by means of a preceding image forming means is separated from the medium, so that the proposed apparatuses cannot be put to practical use.